The present invention relates to a light curing device and method for curing light-polymerizable dental materials. The device comprises a light source with which the light-polymerizable dental materials are cured. Apparatus of this type is known from U.S. Pat. Nos. 6,095,812 and 6,123,545 the subject matter of said patent applications being incorporated herein by reference thereto. A control circuit is provided for controlling light distribution according to a pre-determined output profile. In accordance with this invention the output profile is controlled to have a start-up period, followed by a main curing period.
A number of light curing devices are known which attempt by various means to achieve a curing effect that is as good as possible and attempt to avoid the creation of open margins. It has been known from the printed publication xe2x80x9cK.-J. Reinhardt: Der Einfluss der Lichtquelle auf die Randstxc3xa4ndigkeit von Kompositfxc3xcllungenxe2x80x9d, Carl Hanser Verlag, Mxc3xcnchen, 1991, that light sources of a high light intensity have the propensity to create open margins.
In order to delay the Trommsdorff effect, it has further been suggested to provide a decreased light output value at the start of the entire lighting period in order to ensure that the viscosity of the dental material does not increase too fast.
A further problem of dental materials is their final curing grade (hardness). In order to achieve a satisfactory result, it has already been suggested to increase the light output step-wise over the length of the entire curing duration.
Despite these attempts, the shrinkage value has so far been unsatisfactory with known light curing devices.
Furthermore, light curing devices have been known for a long time with which the light output shall be provided by means of a Xenon flash lamp. With such light curing devices, the duration of voltage application of each light impulse lasts only 0.1 to 2 milliseconds, insofar reference is made, for example, to DE-OS 32 15 664.
Studies with such light curing devices have, however, led to the result that the curing effect is unsatisfactory so that such devices have not found acceptance in practice. Moreover, because of its size, the Xenon flash lamp which is of a usually complicated design is mostly suitable for stationary devices, with respect to which reference is also made to the aforementioned publication.
Therefore, it is an object of the present invention to provide a light curing device of the aforementioned kind with which the curing shrinkage and the temperature induced stress are reduced for curing light-polymerizable dental materials. By means of the low shrinkage value, the marginal integrity (marginal adaptation) shall be improved when curing composites.
This object is solved by the output profile showing a pulsed output with alternating high output value and low output value.
Accordingly, the inventive light-curing device, having a starting time period with an increase in the light output, is combined with a main curing time period during which the light output is alternated at an alternating high and low output value. Surprisingly, this combination results in a decreased curing shrinkage. Apparently, the time period of low light output allows the material to flow against the cavity edge. A gentle curing is achieved, and the propensity for the creation of open margins is reduced by the inventive light curing device. A factor that may also contribute to this result is that the temperature-induced stress is significantly reduced because of the pulsating action in comparison to conventional light curing devices.
While the earlier mentioned studies according to the publication of K. J. Reinhardt show the introduction of stresses due to high light intensity, studies based on the inventive device have surprisingly shown that the inventive short-term high light output is by no means detrimental, but that it favorably affects the hardness of the cured dental material.
A further favorable feature with respect to the invention is that the dental material becomes significantly less warm on curing in comparison to the continuous irradiation at a high light output. During a curing duration of 40 seconds, the temperature increase only amounts to 7xc2x0 C. (when curing 25 mm3 Tetric Ceram, a light-curing composite of the Vivadent company).
It is particularly advantageous according to the invention if the curing application is performed with a not too long alternating period between the high and the low output value. The alternating period can, for example, last for 2 seconds. With alternating periods in this range, the most favorable values result for the final hardness, on the one hand, and the curing shrinkage, on the other hand
An advantageous embodiment of the invention provides to use an incandescent lamp as a light source. An incandescent lamp has a certain inertia with respect to the light radiation. The delay ranges between 100 milliseconds and up to almost one second with larger lamps. Moreover, the internal resistance of a cold spiral filament is significantly higher than the internal resistance of a hot spiral filament, so that usually an electrical connection impulse has to be handled on alternating electrical connection/disconnection pulses, i.e., on changes between an electrically fully connected and an electrically fully disconnected incandescent lamp. This alternation of a connection/disconnection pulse also puts stress on the spiral filament, resulting in a decreased service life.
In order to avoid this result, it is particularly advantageous if a slanted connecting flank is used for the transition between the low light output and the high light output. The fact that the commercially available halogen lights have a light output which is, for example, reduced by 60% at a 30% reduced electrical power can be especially favorably taken advantage of for the present invention. Due to the distinctly lower temperature of the spiral filament at a reduced luminous power, the emission of light is more than proportionally lowered. This often undesired behavior of spiral filaments can be particularly favorably taken advantage of, according to the invention, for the recovery periods of the light-polymerizable dental materials. A reduction of the electrical power by 30%, for example, accordingly results in a reduction of the light output by 60% or 70% which is sufficient for providing the recovery period.
According to a further, particularly favorable aspect of the invention, it is provided to select the starting time as to be comparatively rather long, for example, up to 0 40% of the entire lighting duration, so that the main curing time is 60% of the entire lighting duration. An especially favorable pre-curing effect and a uniform curing effect result therefrom. According to the invention, the core curing can be especially favorably affected by the combination of the relatively long starting time with the pulsating main curing time.